The present invention relates to light-emitting diode (LED) systems. More particularly, embodiments of the invention relate to methods and circuits for short-circuit protection of LED systems.
As electronic products grow rapidly, LCD products, such as televisions and displays, etc., are becoming widespread. As a result, the need for LED backlight and the demand for LED and LED drivers are increasing. Many LED driver integrated circuits utilize multiple-path constant current control to achieve the desired current distribution. In order to obtain stability of system circuit and provide satisfactory current output to the LED, it is necessary to test the voltage conditions of serial LED arrays. The anode of a serial LED array is often connected to a relatively fixed high voltage, while the cathode is often connected to a current control unit. Therefore, testing can be carried out at the cathode of an LED array to determine whether a short-circuit condition exists in the LED array.
FIG. 1 illustrates a conventional LED lighting system including a multiple channel LED array. FIG. 1 shows an LED array 100 having multiple LED strings 110, each having a plurality of LED devices. A constant current controller 101 is connected to a cathode end of each LED string 110. A level shifter circuit 102 is also connected to each of the LED strings. An output of level shifter 102, designated as Vout, provides power to the anode end of each of the LED strings.
FIG. 2 illustrates a conventional method for testing for a short-circuit condition in an LED string. In this method, testing is carried out by sampling the voltage at the cathode terminal of an LED string. In FIG. 2, LEDX denotes a cathode end of an LED string No. X. A voltage Vs taken from a voltage divider formed by resistors R1 and R2 is used as a sampled voltage representing the voltage at LEDX. As shown, Vs is an input to comparator 210, which also has another input coupled to a reference voltage Vref. In this example, Vref is derived from power supply Vdd through a voltage divider formed by resistors Ra and Rb. The resistors and the reference voltage can be selected such that when Vs is higher than or equal to Vref, a short-circuit condition is determined to exist. At this time, the output of comparator 210, V1, will be at a high voltage, for example, to indicate the short-circuit condition.
Although widely used, conventional LED short circuit testing methods, such as those described above, suffer from many limitations. These limitations tend to lead to unreliable testing results, as described in more detail below. Therefore, more efficient and cost-effective techniques for improving LED systems are highly desired.